Purification of lanthanum salts



Patented Feb. 12, 1946 UNITED STATES PATENT.OFFICE Lawrence E.Martinson, Rochester, N. Y., assignors to Eastman Kodak Company,Rochester, N. Y., a corporation of New Jersey No Drawing. ApplicationAugust 19, 1944, Serial No. 550,312

2 Claims.

This. invention relates to the purification of rare earth compounds andto glass made therefrom. More particularly, it relates to thepreparation of lanthanum oxide (LazOa) suitable for preparing'color-iree optical glasses. While this application parallels in somerespects, our previous Patent No. 2,364,613, issued on December 12,1944, it is an improvement thereover in the oxidizing agent employed andth manner of employing it.

The sources of lanthanum are the various rare earth containin mineralssuch as the monazite sands from which the lanthanum is separated bymethods which are described in some detail in the literature. While thelanthanum can be obtained in relatively pure salt forms by repeatedrecrystallizations, these salts ordinarily still retain small amounts ofimpurities in the form of iron salts and the more closely related ceriumsalts. The elimination of the last traces of these and other impuritieshas been accomplished heretofore only with considerable difliculty andex.-' pense. Ifhis hasbeen particularly tru in the case of the solubledouble salt of ammonium lanthanum nitrate, which fact has restricted its25 usefulness as a source of lanthanum oxide in the art of makingcertain optical glasses. It has been our experience that the presence ofeven very small traces of iron and cerium in the lanthanum oxide resultsin glasses having a distinctly yellow or amber color. For example, anamount of cerium oxide (CeOz) no greater than 0.1% by weight based on thweight of the glass, gives a yellow colored glass with materials whichnormally in the absence of any cerium would yield a practicallycolorless product. Iron impurities give amber colored glasses. It willbe appreciated that the presence of appreciable color in certain opticalglasses would make such glasses impractical of use in systems where theabsence of color is a critical requirement. We hav now found that thisdisadvantage can be overcome by our new process of purification.Lanthanum salts such as the acetate, bromate, bromide, chloride,nitrate, and the like, which contain iron and cerium as impurities, canbe rendered sufliciently free of these elements by our process so thatthe lanthanum oxide and the glasses de-' ject is to prepare lanthanumoxide which is suitable for making practically colorless optical lass.

In the preferred method of carrying out of the process of our invention,ordinarily we start with the soluble double salt of ammonium lanthanumnitrate, as obtained by the usual extraction methods from monazite sandsand purification by repeated recrystallizations, dissolve it in water,

to approximately a 25 per cent solution (ratio of weight of salt tototal weight of all ingredients of the solution), add solid ammoniumcarbonate. and heat the solution close to the boiling point,

add ammonium persuliate and heat the final solution to boiling. At first(upon adding the ammonium carbonate) some white lanthanum carbonate isprecipitated, but as the solution approaches the boiling point and theammonium persuliate is added, there isa gradual additional precipitationin the form of a light yellow, easily filterable, precipitate of ironand cerium. After settling for a short time, the solution is filtered,and the filtrate either evaporated to dryness and the residue igniteddirectly to the lanthanum.

oxide, or preferably, the filtrate is treated with oxalic acid and theprecipitated lanthanum oxalate filtered off and ignited to the lanthanumoxide. The latter is employed directly in ad- In our process, theammonium per-sulfate converts any cerium or iron to the higher oxidizedform and need be employed in excess only slightly greater than thatrequired for this conversion. The ammonium carbonate neutralizes thenitric acid, or any other mineralacid, which may be present in theparticular lanthanum salt to be purified, and is employed in an amountsuflicient merely to raise the pH to the point necessary to neutralizeall the mineral acid and to precipitate 4,5 a small amount of lanthanumcarbonate which can be returned to the system, in the next-batch to bepurified. The iron is precipitated as a hydroxide, whereas the cerium isprecipitated in some complex form of easily filterable hydroxide andcarbonate. The reaction itself will not take place at ordinary roomtemperature, but only at temperatures approaching the boiling point ofthe solution, i. e. in excess of C. A distinct advantage possessed byour process is that all the reagents employed ar volatile and easilyeliminated in the subsequent step of ignition oi the lanthanumoxalate tothe oxide.

The invention is illustrated further by the following example:

Example 300 grams of double ammonium lanthanum nitrate salt(La(NO3)3.2NI-I4NO3.4H2O) containing cerium equivalent to 0.14 per centCeOz were dissolved in 1000 cc. of water; 2.9 grams of ammoniumcarbonate were added with stirring. The solution was heatedto 90 0.; 5gramsof amemonium persulfate were then added and the The latter wasfilteredofl and ignited, thereby yielding a very pure lanthanum oxide. IA glass prepared from a mixture consisting on a weight basis of 26% ofthe above prepared lanthanum oxide, 18% tungsten oxide, 12%thoriumvoxide,

12%. barium oxide and 32% boric, oxide was; practically colorless,whereas the glass control sample prepared in the same way, but employinglanthanum oxide prepared without the benefit of the intermediatestep'oi, our invention from the same doublesa'lt was very yellow incolor.

While our invention has been illustrated more particularly with a doublesalt of ammonium lanthanum nitrate containing cerium equivalent todizing agent, and more specifically in adding the oxidizing agent to thehot solution of the other ingredients. While the use of hydrogenperoxide accomplished its intended purpose, it was found that upon largescale operations it was sometimes necessary to repeat the oxidation andfiltration, whereas when employing ammonium persulfate and particularlywhen adding it to the hot solution of the-other ingredients, completeprecipitation of the iron and cerium impurities took place in oneoperation without any necessity to repeat the process. The ammoniumcarbonate and ammonium persulfate may, of course, be addedsimultaneously to the cold solution and the solution then heated or maybe added to the hot aqueous solution of the lanthanum salt and then thesolution further heated, but better. results seem to be obtained if theammonium carbonate is added first so as to insure substantial neutralityof the solution,'before adding the ammonium persulfate, the latter beingpreferably added to the solution while hot and further heated.

We claim:

l. The process of removing iron" and cerium from water-soluble lanthanumsalts which scinprlses treating an aqueous solution of such a salt vwith sufiicient ammonium carbonate to neutralize 0.14 per cent CeOa asan objectionable impurity,

and is primarily concerned with quantities of cerium and iron of thisorder or even less, it will be understood that our process is likewiseapplicable if desired, to the purification of soluble lanthanum saltscontaining many times this amount of impurities. For larger amounts ofiron and cerium, it is only necessary to increase the quantity ofreagents employed. .Also the solution concentration of the salt to bepurified is not at all critical, since concentrations as'low as 5 percent or even less, as well as concentrations somewhat higher than 25 percent can be successfully employed, such higher concentrations beinglimited only by the solubility of the particular lanthanum salt to beThe improvement constituting the instant invention distinguishes fromthe invention of our prior application in the use of ammonium persulfateinstead of hydrogen peroxide, as the oxiany free acid ions presen'tandto just start to precipitate lanthanum carbonate, adding to suchsolution an excess of ammonium persulfate over the amount required toconvert the iron and cerium ions present to the higher oxidized state,heating the solution to a'temperature approaching the boiling pointthereof and filtering oil the insoluble compounds of iron and cerium soformed.

2 The process of removing iron and cerium from the double salt ofammonium lanthanum nitrate which comprises treating an aqueous solutionof such salt with suflicient ammonium carbonate to neutralize any freeacid ions present and to just start to precipitate lanthanum carbonate,heating the solution to a temperature above C., then adding to suchsolution an excess of ammonium persulfate over the amount required toconvert the iron and cerium ions present to the higher oxidized state,heating the solution to approximately the boiling point thereof andfiltering on the insoluble compounds of iron and cerium so formed.

ALBERT. E. BALLARD. LAWRENCE E. MAR'I'INSON.

then the solution being

